Staphylococcus aureus is a leading cause of community and nosocomially-acquired infectious and toxin-mediated syndromes, some life threatening, that affect patients of all ages. The glycopeptides (GP) have been the most reliable alternatives for the therapy of S. aureus isolates that are resistant to methicillin, cross resistant to all beta-lactams, and often resistant to a wide spectrum of unrelated antimicrobials. However, the effectiveness of GPs has been eroded by the increasing recognition of resistant isolates. Our ongoing studies are aimed at identifying GP resistance mechanisms. Despite the description of numerous phenotypic and biochemical characteristics among resistant isolates, the mechanism(s) of GP resistance in S. aureus has remained incompletely defined. Available data suggest that acquisition of the resistance phenotype involves cell wall reorganization; pleiotropic changes have been documented such as altered peptidoglycan structure, coagulase activity, binding of vancomycin, autolytic activity and lysostaphin susceptibility. However, it seems unlikely that a single mechanism or sequence of mechanisms will account for resistance in all clinical glycopeptide-resistant isolates studied to date since no phenotypic or biochemical change has been uniformly found. We believe that the resistant phenotype involves multiple genetic changes. We plan to investigate the mechanism(s) of resistance with a multi-pronged approach. First, with the complete genomic sequence of four S. aureus isolates at hand, we will employ microarray analysis to compare expression patterns of relevant cell wall metabolic and 2-component signal transduction genes between GP-susceptible and resistant isolates. The availability of isogenic susceptible and resistant clinical isolate pairs will provide invaluable tools for this analysis. Appropriate up and down regulated genes will be targeted for further investigation including sequence comparison, Northern blot analysis, allelic inactivation and overexpression in relevant genetic backgrounds. These studies should lead to an understanding of the mechanisms by which S. aureus resist the bactericidal effect of GPs and hopefully can identify new ideas regarding therapy of infections caused by resistant isolates.